The present invention relates to a power-assisted steering system for use in wheeled vehicles, and more particularly to a control apparatus for the power-assisted steering system which is arranged to control a hydraulic power assist to the driver's steering effort in accordance with a travel speed of the vehicle and/or a rotary angle and a rotational speed of the steering shaft.
Conventional power-assisted steering system are arranged to increase the hydraulic power assist in accordance with an increase of the rotary angle of the steering shaft so as to ensure stability of the steering wheel in its neutral position during high speed travel of the vehicle. It has, however, been experienced that when the steering wheel is fully rotated during low speed travel of the vehicle on a road, the driver's operational feel deteriorates because of an increase in the hydraulic power assist. If the hydraulic power assist is decreased in accordance with an increase of the rotary angle of the steering wheel during low speed travel of the vehicle, the driver's feel during rapid turning would deteriorate because of insufficient hydraulic power assist. Additionally, in operation of the conventional power-assisted steering system, as shown by the reference character L in FIG. 7, the manual torque necessary to effect the hydraulic power assist during high speed travel of the vehicle is substantially the same as that during low speed travel of the vehicle. For this reason, the steering wheel may not be sufficiently weighted during high speed travel of the vehicle.